I draw your attention to subscription and unsubscribe.
I would call both of these the very definition of a meta tag. "Subscribing" could be to a mailing list (or similar), paying for a service, or just "watching" for some sort of serially released material. "Unsubscribe" is just the opposite action.
Further, there is no tag wiki or usage guidance for either.
Currently 18 questions for subscription.
3 about Google Groups1 about Office 3653 about YouTube1 about Google Play Music1 about Twitter1 about Trello1 about Posterous1 about Facebook1 about GitHub1 about Feedburner1 about SourceForge1 about Blogger1 about Google Calendar1 about links in spam email
Currently 7 questions for unsubscribe.
1 about Backblaze1 asking about a "reputation database" for "unsubscribe links" in email messages1 about email messages from Facebook1 about notifications from Facebook2 about Trello1 about a site called "Find Me Spot"
Neither tag could possibly stand on its own. Further, no question gains any extra clarity beyond the app tag(s) that are already on the question.
These tags should be "burninated".